As is well known, diamond is the hardest substance known and is therefore difficult to polish. Other materials can be polished with harder substances, but diamond is polished only with diamond in the form of diamond powder or "grit". Polishing is necessarily slow and a large amount of diamond abrasive powder is consumed.
Polishing is desirable not only on natural gem stones but also on synthetic and polycrystalline diamond used for industrial purposes. For example, bearings have recently been devised with both bearing faces being made of polycrystalline diamond. The diamond faces are complementary and are polished smooth to take advantage of the low coefficient of friction of diamond-on-diamond (in the order of 0.02).
Polishing of a pair of complementary conical bearing surfaces can be considered as an example of the shortcomings of prior polishing techniques and an advantage of this invention. Each of the bearing surfaces comprised areas of polycrystalline diamond that collectively formed a conical surface, one external cone and one internal cone. The bearings were ground and polished by relative rotation with the bearing surfaces engaged, using diamond grit as an abrasive between the bearing surfaces. As much as 0.2 millimeters (0.005 inch) of material was removed from some areas of each diamond surface. Such polishing by conventional techniques took as long as three weeks for each bearing pair and about 25 carats of diamond grit was consumed for each 6.5 square centimeter (one square inch) of polished surface. Substantial costs are involved, not only for the diamond grit but also for the polishing equipment used for such a long interval.
By comparison a similar bearing has been burnished according to this invention in less than one hour with no consumption of diamond grit. This was accomplished with manual adjustments and is believed that with automatic control of burnishing the total time for polishing can be measured in minutes instead of in weeks.
As mentioned above, polishing is desirable for polycrystalline diamond surfaces. Polycrystalline diamond is usually, but not necessarily, made from synthetic diamonds rather than natural diamonds. Synthetic diamonds are typically made by subjecting graphite to high temperature at a sufficiently high pressure that diamond is the thermodynamically stable crystal structure for carbon. Conversion of graphite to carbon is preferably conducted in the presence of a catalytic metal such as cobalt. Diamond can be synthesized at a pressure of 65 kilobars and 1500.degree. Kelvin. Various other pressures and temperatures can be used as is well known to those skilled in the art.
Depending on the operating parameters during synthesis of diamond various crystal sizes of diamond can be produced. Much of the diamond is in the form of individual crystals, although twins and other polycrystalline forms are not uncommon.
Polycrystalline diamond for industrial purposes such as bearings can be made by subjecting a mass of diamond crystals to high temperatures and pressures for sintering the diamond and producing diamond-to-diamond bonds between crystals. Such sintering can be without catalyst as described by H. Tracy Hall in Science, Volume 169, Aug. 28, 1970, pages 868 and 869.
Alternatively, diamond can be sintered in the presence of a catalytic metal such as cobalt as described in U.S. Pat. No. 3,141,746. Other techniques for forming polycrystalline diamond can also be practiced. If desired, a layer of polycrystalline diamond can be formed on a cemented carbide backing. Such product is desirable for bearings and for a variety of other industrial applications where the strength and stiffness of the cemented carbide helps support the diamond layer.
Typically, conventional polishing commences with relatively coarse hard grit which continually scratches the surface of the material being polished until all of the scratches remaining on the surface are as small as can be made with that size grit. The next step is to polish with a smaller size grit until all of the larger scratches are removed and the only remaining scratches are the smallest that can be produced with this second size grit. This continues with successively smaller grit sizes until the desired degree of polishing is obtained.
At this stage, the largest scratches remaining in the surface are about half the size of the grit being used. For polishing diamond, the smallest practical grit size is about one micron. Thus, a typical well-polished diamond surface has one-half micron scratches.
"Grit" refers to discrete particles of abrasive in a chosen size range used for polishing. For most purposes, the grit is harder than the material being polished. When diamond is being polished, the grit is diamond powder. Polishing grit is continually reduced in size during polishing and must be replenished as it is consumed. For most polishing the grit is "loose", that is, it is not attached to either surface and can tumble or slide in the polishing interface. During lapping the grit is pressed into the surface of the lap so as to be more or less held in place and caused to slide across the surface being polished. The particles become blunted as edges break or wear away and the polished surface has rounded scratches.
Another common characteristic of polished surfaces is waviness. Waviness is a periodic or aperiodic wavelike variation from a perfect surface which is generally much larger and wider than the roughness in the form of scratches left by grinding or polishing. Depending on the application of the product, waviness may be undesirable while minute scratches can be tolerated. For example in gauge blocks, the polished steel surface has little waviness but on a microscopic scale is scratched. Polished metallographic specimens may have moderate waviness but are commonly etched to provide a scratch-free surface.
Diamond can be roughly cut by a technique known as bruting. This involves cutting a diamond by rubbing it with another diamond or diamond chip. There is considerable chipping of the diamond surface and a very rough surface is obtained, which is then polished by conventional grit polishing.
A technique where the surface is polished by rubbing with a hard smooth object can be used for polishing some materials such as metals. This is often referred to as burnishing. For example, silver or leather can be burnished with a steel tool. In this form of burnishing there is plastic flow of the material being polished. Material is not ordinarily removed from the burnished surface, the surface is merely "rearranged". When done with clean tools, the surface may retain appreciable waviness but be largely scratch-free. Burnishing diamond has not been previously considered since there is nothing harder than diamond or with greater compressive strength, nor is diamond subject to plastic flow at anything approaching practical pressures.